Five differences that form the basis for chromosome aberrations division into exchanges and fragmentations are described. A fragmentation is a more variable component of aberrations that sometimes exhibit the properties of incompleteness. The period of exchanges formation is determined. In most objects, it lasts for about 4 h on an average. Osteichthyes have additional long-term patterns of cytogenetic changes occurrence which, in all appearance, favor the evolutionary processes in this group of vertebrates.